Electric heat-regulating switches



July 2, 1957 R. CQLLIER 2,798,132

ELECTRIC HEAT-REGULATING SWITCHES Filed "March w,V 1956 United States Patent iO ELECTRIC HEAT-REGULATING SWITCHES Richard Collier, Esher, England Application March 16, 1956, Serial No. 572,071

Claims priority, application Great Britain March 2, 1956 3 Claims. (Cl. 20G-,122)

This invention concerns heat-regulating electric switches to make and break a load circuit intermittently under control of a heat-sensitive element (e. g. a bimetallic strip) heated by the load current so that the switch operates cyclically, the ratio between the intervals duringV which the switch is open and closed being variable (for instance, by a control knob adjusting the setting of the bimetallic strip) whereby the average rate of supply of energy to the load circuit is Varied. Such a switch is to be understood by the term a switch as hereinbefore referred to.

The object of this invention is to provide an improved switch having positive make and break. A further object is to provide a switch wherein the effects yof variations of ambient temperature on the operation of the switch are compensated for.

According to this invention a switch as hereinberfore referred to comprises a heat-sensitive element which is heated by the electric current to perform a cyclic movement towards and away from a contact member thereby to close and open the heating circuit, said contact member, which is urged to move towards the heat-sensitive element, being held from said movement by a tensioned conductor through which the heating current passes to the contact member, the arrangement being that at ambient temperatures the conductor has a negligible eX- tension and under the heating etect of the current the extension is rapid and considerable.

Preferably the setting of the heat-sensitive element relatively to the contact member is adjustable by an auxiliary heat-sensitive element in response to changes in ambient temperature.

A practical application of the invention will now be described, by way of example only, with reference to the accompanying drawing whereof:

Figure 1 is a front view ot a switch according to the invention with the cover plate removed, and

Figure 2 is a section on the line II-II of Figure 1, the cover plate being shown in position.

Referring to the drawing: the switch is contained in a casing 5 having a removable cover plate 6 through which passes a spindle 7 carrying a control knob 8.

A frame 9 is secured to the casing 5 and has a fulcrum plate 10 upon which is pivotally mounted a bimetallic strip 11, the latter being urged by spring 12 against plate 10 and carrying a cam follower 13 to engage face cam 14. Cam 14 is on spindle 7 and upon rotation of knob 8 the angular setting of strip 11 is varied.

A main moving contact 15 is carried by a heat-sensitive element in the form of a bimetallic strip 16 having a heater winding 17, kthe strip 16 (the main bimetallic strip) being secured to the bimetallic strip 11 (the auX- iliary strip).

A member 18 carries a contact 19 with which contact 15 engages (as later described) the member 18 being of a springy nature so that it is urged towards strip 16 for engagement of contacts 15, 19. A metal wire (e. g. of

nickel silver) 20 is anchored to a terminal 21 and to the member 18 near contact 19 and the wire 20 holds the member 18 from movement to bring contacts 15, 19 into engagement. The wire 20 constitutes the tensioned conductor referred to above.

The wire 20, the contacts 15, 19 and the heater winding 17 are in the electric circuit of the heating apparatus, which in the particular arrangement being described is a hot plate for an electric cooker. The wire 20 is so selected as to size and resistance that for a given current to the hot plate the wire attains a temperature greatly in excess of ambient temperature e. g. 500 F. as compared with a maximum ambient of 200 F. With a hot plate of 1800 watts a wire" 20 of nickel silver .0022 diameter has been foundsatisfactory in practice. By disposing the wire 20 on the side of member 18 which is remote from the strip 16 it is little affected by the heat from winding 17.

In use: the hot plate being inoperative knob 8 is adjusted whereby cam 14 displaces the auxiliary and main bimetallic strips whereby contact 15 engages contact 19 and the'circuit to the hot plate is completed. The winding 17 becomes heated and the bimetallic strip 16 moves to disengage the contacts. When this occurs the winding cools ot and the sequence is cyclically repeated. The distance between the contacts 15, 19 being selected by cam 14 it is possible to vary the ratio between the intervals during which the circuit to the hot plate is open and closed and thus to vary the average rate of supply of energy to the hot plate.

Now during this operation it is desirable that a positive make and break be made at contacts 15, 19. The wire 20 is provided to this end.

When the contacts 15, 19 engage the current tlows through wire 20 which becomes heated and expands and member 18 is urged towards strip 16 so that adequate contact pressure is maintained. When contact 1S moves away from contact 19 due to displacement of strip 16 the wire (because it has very small bulk and thus no heat storage capacity) immediately cools and draws contact 19 away as contact 15 moves oit. A positive break at the contacts results.

With cooling of winding 17 contact 15 moves towards contact 19 and when, at the first instant, the contacts lightly engage, the wire rapidly heats up and the contact pressure becomes considerable so that a positive and effective make results at the contacts.

As shown in Figure 1 there is a switch 21a having a movable arm 22 provided with a projection 23 to enter detents'24 in the side of the cam 14. Each detent corresponds with a particular hot-plate temperature and the cover is marked l, 2, 3, 4, 5 and full to correspond. There is also an oit position. When knob 8 is set opposite a selected mark the projection by engagement in the appropriate detent will mechanically position and hold the cam at the required setting. In this way at each selected setting the housewife is assured of obtaining identical cooking conditions from one time to the next. Of course settings between those marked may also be readily obtained.

For positions off and 1 to 5 the detents are such that switch 21a is inoperative. At the full position the detent 24a is of greater depth and accordingly when full heat is required switch 21a is closed. Switch 21 is thus placed in circuit to the hot plate, current being supplied thereto through the switch which short-circuits the switch comprising contacts 15, 19.

Instead of a bimetallic strip 16 a thermostat may be used.

The auxiliary bimetallic strip 11 is responsive to ambient temperatures and varies the position of strip 16 with changes in temperature. Such control may be arranged to be sensitive and effective because the control is not affected by the wire 20 since, as indicated above, the latter is not responsive to ambient temperature to any appreciable extent.

1 claim:

l. A heat-regulating electric switch as hereinbefore referred to comprising in combination a heating circuit incorporating a main bimetallic strip and a springy member, co-operating electric contacts carried respectively by said strip and member, a heater winding associated with the main bimetallic strip for electing a cyclic movement of this strip towards and away from the springy member to engage and disengage said contacts to close and open the heating circuit, an auxiliary bimetallic strip responsive to ambient temperatures associated with the main bimetallic strip for varying the position of the latter relatively te the springy member according to changes in ambient temperature, a conductor wire anchored at one end to a terminal and at its other end to the springy member adjacent the Contact carried thereby, whereby said wire is heated when current llows therethrough and expands and elongates to enable the springy member to move towards the main bimetallic strip to provide adequate pressure between the contacts, and contracts when the current flow is interrupted to move the springy member away from the main bimetallic strip.

2. A heat-regulating electric switch as hereinbefore referred to comprising in combination a heating circuit J incorporatlng a main bimetalllc strip and a sprmgy member, co-operating electric contacts carried respectively by said strip and member, a heater winding associated with the main bimetallic strip for effecting a cyclic movement of this strip towards and away from the springy member to engage and disengage said contacts to close and open the heating circuit, an auxiliary bimetallic strip responsive to ambient temperatures associated with the main bimetallic strip for varying the position of the latter relatively to the springy member according to changes in ambient temperature, a manual control knob for setting the main and auxiliary bimetallic strips relatively to the springy member, and mechanical means for holding the knob adjusted in any one of a number of different positions, including a position in which the main bimetallic strip is rendered inoperative, a conductor wire anchored at one end to a terminal and at its other end to the springy member adjacent the contact carried thereby, whereby said wire is heated when current flows therethrough and expands and elongates to enable the springy member to move towards the main bimetallic strip to provide adequate pressure between the contacts, and contracts when the current flow is interrupted to move the springy member away from the main bimetallic strip.

3. An electric switch in accordance with claim 2, comprising a cam follower carried by the auxiliary bimetallic strip, a rotatable cam supported for engagement with said follower and actuated by the manual control knob, and an added switch controlled by said cam, said added switch being included in the heating circuit and being closed when the heat-sensitive-element is rendered inoperative.

References Cited in the tile of this patent UNITED STATES PATENTS 2,272,976 Geer Feb. 10, 1942 2,436,909 Werner Mar. 2, 1948 2,646,479 Ashe July 21, 1953 2,716,173 Russell Aug. 23, 1955 2,727,131 Russell Dec. 13, 1955 FOREIGN PATENTS 743,089 France Jan. 6, 1933 

